A service registry and repository registers and stores service descriptions as part of a service-oriented architecture (SOA) promising business agility and resilience through reuse, loose coupling, flexibility, interoperability, integration and governance. These promises are addressed by separating service descriptions from their implementations, and using the service descriptions across the life cycle of the service. Standards-based service metadata artefacts, such as Web Service Definition Language (WSDL), XML schema, policy or Service Component Architecture (SCA) documents, capture the technical details of what a service can do, how it can be invoked, or what it expects other services to do. Semantic annotations and other metadata can be associated with these artefacts to offer insight to potential users of the service on how and when it can be used, and what purposes it serves.
Service descriptions are used by analysts, architects, and developers during a Development Phase of the SOA life cycle to locate services to reuse and to evaluate the impact of changes to service configurations. Service descriptions are used by deployers in a Change and Release Phase and by administrators in a Runtime Integration Phase. It is used in the Operation Phase of the life cycle to support policy enforcement required by Service Level Agreements (SLAs) and to present a more comprehensive view of the managed service environment. Service descriptions are used by various technologies for different purposes and needs to be persisted in a reliable and accessible format. Service descriptions are variously described as metadata, objects, documents, entities and artefacts.
A service registry and repository is an application that persists service descriptions as a data objects in a relational database system. Typically a user uses a graphical tool to design classes that represent the service descriptions that need to be persisted. Java objects, that represent the classes, are compiled into binary files. Database schema is generated that can represent the objects and is installed into a relational database (IBM®DB2®, Oracle® etc). The objects are registered with service register and stored within the repository. The objects are persisted and retrieved directly from the object database. IBM and DB2 are registered trademarks or trademarks of International Business Machines Corporation in the United States and/or other countries. Oracle is a registered trademark of Oracle Corporation.
One type of service registry and repository stores the descriptions as objects within the repository and queries the repository using an object query language. An example of this type of service registry, IBM WebSphere® Service Registry and Repository (WSRR), uses an IBM object query language called XMeta Query Language (XMQL) for internal communication with the repository. WebSphere is a registered trademark or trademark of International Business Machines Corporation in the United States and other countries. XMeta Query Language (XMQL) is a declarative object query language designed for the efficient retrieval of objects stored in a repository such as WSRR. XMQL allows the use of the dot operator for accessing predefined properties on a given modelled object type. XMQL also allows a dereferencing operator for traversing relationships between any two given object types. The XMQL query expression is defined in the context of a given package registered with WSRR. XMQL has a “SELECT-FROM-WHERE” type of structure and returns a set of objects.
Patent publication U.S. Pat. No. 7,028,028 (Enosys Markets Inc.) discloses a system for querying markup language data stored in a relational database according to markup language schema. The system translates from the input XML query language to apply corresponding query instructions to the underlying relational database using a relational database query language such as SQL.
Although a relational database query language is a useful way to access objects in a repository it does not return complex collection of objects as an object query language can. As such, an object query language is a powerful way to access objects in a repository. Furthermore, object query languages provide additional features such as object references and dot notation where SQL does not. However, one problem for users accessing objects in the repository is that a specialized object query language can be counter intuitive if one is used to a different query language such as a path expression query language. XPath is a path expression language that is more suitable for accessing content within an XML instance document than for accessing objects in an object database but many users are familiar with it and prefer to use it to locate objects.
XPath defines a query using a path to point to a type of object, if the XPath query further defines the objects it wants to select by a property or relationship of a subtype but not necessarily the that type then it uses a treat-as expression. For instance, the XPath expression for selecting 3 wheeled Italian cars would be defined as selecting from objects that are Italian vehicles that can be treated as cars with three wheels. This is because three wheels is a property of cars but not necessarily vehicles and Italian is a property of vehicles. It would be advantageous to be able to use an XPath treat-as expression to access objects in a service registry and repository.